1. Field of the Invention
The present invention relates to an optical system and an image pickup apparatus incorporating it and, more particularly, to those suitably applicable to image pickup apparatus of cameras, video cameras, still video cameras, etc. adapted to form an object image on a surface of an image pickup element by use of an optical system having off-axial reflecting surfaces a normal to which does not coincide with a reference axis.
2. Related Background Art
Viewing optical systems and optical systems for distance measurement and photometry used in the cameras, video cameras, etc. are often constructed so as to make use of rays passing through an image pickup optical system in order to avoid the influence of parallax. This method is generally called the TTL method.
On the other hand, there are conventionally known optical systems making use of an optical element of one block including many reflecting surfaces. Examples of such optical elements are a pentagonal roof prism, a Porro prism, etc., used in the finder system of a camera, and optical prisms, such as color separation prisms or the like, for separating light from a taking lens, for example, into three-color beams of red, green, and blue and forming an object image based on each color beam on a corresponding image pickup element surface.
An optical system of a single-lens reflex camera using the pentagonal roof prism will be described as a typical example of the optical system of the TTL method using an optical prism, referring to the sectional view illustrated in FIG. 1.
In FIG. 1, rays passing through the image pickup optical system 101 are reflected by a reflecting mirror 102 to form a primary image of an object on matt 103. After that, the light is guided via a viewing optical system composed of a condenser lens 104, an erecting prism (pentagonal prism) 105, and an eyepiece 106, to the eye. During photography, the reflecting mirror 102 is retracted out of the optical path, and an object image is formed on an image pickup surface 108 on which a photosensitive film, an image pickup element, or the like, is placed. The light reflected by the reflecting mirror 102 is also utilized for distance measurement and photometry in some cases.
The structure illustrated in FIG. 1, however, requires a sufficiently large space for the reflecting mirror 102 to be set in the optical path. For that reason the image pickup optical system is needed to have a large back focus, which increases the number of lenses in the image pickup optical system and increases the size of the image pickup optical system. The reflecting mirror 102 heretofore was a plane mirror having no optical power and was not utilized for correction for aberration.
Meanwhile, it has been clarified recently as to non-coaxial, optical systems that an optical system of well-corrected aberration can be constructed by introducing the concept of the reference axis and making its component surfaces of asymmetric, aspherical surfaces; for example, Japanese Patent Application Laid-Open No. 9-5650 discloses a design method thereof and Japanese Patent Applications Laid-Open No. 8-292371 and Laid-Open No. 8-292372 (both corresponding to U.S. Pat. No. 5,825,560) describe design examples thereof. Such non-coaxial, optical systems are called off-axial, optical systems (which are optical systems defined as optical systems including a curved surface (off-axial curved surface) a normal to which at an intersecting point of the component surface with the reference axis does not lie on the reference axis, where the reference axis is assumed to run along a ray passing the center of the image and the center of the pupil, the reference axis being of a bent shape).
In these off-axial, optical systems, the component surfaces are normally non-coaxial and do not bring about an eclipse even if they are reflective surfaces. Therefore, it is easier to construct an optical system of reflective surfaces. They also have such features that routing of the optical path is relatively free and that it is easier to construct an integral optical system by the technique of integrally molding the component surfaces.